The "add-on line" does not have all the same functionality as the main line due to its low cost. In order to forward a number, you would need it to be the primary line (or have it ring to the primarily line and forward the whole account).

The reason for this is that they are offered at a deep discount and forwarding uses double the minutes (we have to pay for the incoming call and an outgoing call to the destination you forward to for the entire length of the call). If we allowed individual forwarding on the discounted virtual numbers, people could in theory add a bunch of them and forward them all independently. It sounds like you found a backdoor way to do it, but the reason it is missing is that we just can't allow discounted virtual numbers to have the same functionality as a full account.

Another option besides making it a full account, would be to move it to our cloud offering which is a forwarding only service that includes 300 minutes per month and is 2.5 cents per minute beyond 300. Here is some information: http://www.voipo.com/cloudnumbers.php